HVAC Duct Work Standards

Contractors

• According to consumer reports, the best way to determine the ducts needed by the HVAC systems in a private home is to hire a contractor. He can then determine the correct duct sizes and runs from Manual D of the Air Conditioning Contractors of America. This document takes into account the climate, the home’s design, insulation, location, the number of openings like doors and windows, whether walls are shaded or exposed to sunlight and the number of occupants. Similar calculations for the size of the HVAC system are available from Manual J of the same source.

Above-Ground

• Equipment connected to above-ground HVAC ducts must not discharge air hotter than 250 degrees Fahrenheit. Round ducts made from galvanized metal must have a minimum thickness between 0.0157 and 0.0236 inches. If made from aluminum, minimum thickness must be between 0.0145 and 0.023 inches. Exposed rectangular ducts can have a minimum thickness of 0.0157 to 0.187 inches if made of galvanized metal, and 0.0145 to 0.018 if made from aluminum. Any materials used for duct systems must have a flame spread index of 200 or less.

Underground

• Underground duct systems must be made out of approved concrete, clay, metal or plastic materials. If they are not metallic, they must be installed according to the manufacturer’s installation instructions. Plastic ducts cannot have maximum temperatures above 150 degrees Fahrenheit. Metal ducts need an approved protective coating and must have corrosion protection or be encased concrete at least 2-inches thick. Any ducts to be encased in concrete need to be sealed and secured before the concrete is poured.

Insulation

• Duct insulation, linings, coverings and adhesives need a flame spread index of 25 and under, and a smoke-developed index of 50 or under. Ducts in attics and crawl spaces can accept sprayed application of polyurethane foam only if the flame spread is under 25 and the smoke-developed index is under 450. They must not glow, flame, smolder or smoke when tested at temperatures above 250 degrees Fahrenheit. Any external duct insulation and factory-insulated flexible ducts need a legibly printed label every 36 inches. This label must contain, at minimum, the name of the manufacturer, the R-value at the specified installation thickness, the flame spread index and the smoke-developed index.